double exponential decay and tau calculation

Hi

I apologize in advance if this really belongs in the "pre-university section"

I have quite a bit of data that I need to analyze. When I use GraphPad Prism 4 and apply a nonlinear regression two phase exponential decay (equation:
Y=Span1*exp(-K1*X) + Span2*exp(-K2*X) Plateau
Starts at Span1+Span2+Plateau and decays to Plateau with rate
;constants K1 and K2. The half lives are 0.69/K1 and 0.69/K2) I get 2 half lives that don't appear to make sense.

When I use Sigma-Plot 10 (either Double 4 paramenter Y=y0 + [a*(e^(-bx))] or Double 5 parameter Y=y0 + [a*(e^(-bx))] + [c *(e^(-dx))]) I get values that again don't make sense and do not equal any value in graphpad.

essentially, what I have are decay data from calcium imaging experiments that have 2 time constants, one that should be very fast at the initial decay phase and one that should be longer at the second decay to baseline.

I have no idea how to obtain the tau (time constants) in either of these stats programs or how to transform the equations above to see why the programs are giving different values.